Method for producing wood-plastic composite material

ABSTRACT

The invention relates to a method for producing a wood-plastic composite material, by which means the composite material is provided with a pre-determined color and pre-determined material characteristics during the course of the method. The method according to the invention is characterized by the drying and treatment of the wood chips using chromophore substances and/or active substances in a fluidized bed appliance during one of the method steps preceding the extrusion. Following the treatment, sorted wood chips can be removed from the fluidized bed within a pre-selected dimension spectrum and extruded with plastic in the subsequent method step.

The invention relates to a method for producing a wood-plastic composite material, wherein a predetermined color as well as predetermined material properties can be provided to the wood-plastic composite material during the course of the process.

When used as intermediate or finished products in the field of construction, furniture manufacturing or the automobile industry, wood-plastic composite materials should have certain colorations and/or material properties, for example, they should resist rot and pests, be fire- and flame-retardant, have UV resistance, and repel water. In addition, haptic properties and/or appearance of a wood-based material are frequently also desired.

Wood-plastic composite materials are currently produced by using an extrusion technique. For extruding the wood-plastic composite materials, the wood chips, the plastic material and additive substances that change coloration and/or material properties are supplied to the extruder and subjected to extrusion. This solution is described, for example, in DE 100 28 981 A1. The wood components are here dried and mixed with the additional required additives, such as adhesive and hardener, and the desired products are demolded exclusively in parts of the extrusion technique.

After extrusion with the pressing tool, the surface of the extruded products is composed, as a result of the process, mostly of plastic, i.e., the extruded products have the appearance and haptic properties associated with an almost pure plastic product.

In order to attain the frequently desired appearance and haptic properties of a wooden material, the surface of the extruded parts is subsequently—in subsequent processing steps—roughened or slightly removed, until a layer that more closely resembles wood can be inferred from appearance and touch. In these processing steps, the wood chips are processed in a way that even the innermost regions are accessed and cut faces of the wood chips become a surface. For this reason, additive substances that provide color and/or change the material properties, such as flame retardants, protection against rot and swelling, pesticides and color-forming substances, are introduced into the wood chips as deeply as possible, in order to be present and active also in newly produced surfaces when the chips are split. Complete penetration of the chips with the aforementioned substances would be optimal.

The structure and function of the extruders must be designed for demanding operation due to the different densities and other properties of the additive substances. The inlet and mixing zones must be specially designed and dimensioned so as to achieve optimal mixing of the chips and additive substances in the pressurized zone, which ensure deep and uniform penetration of the additive substances into the chips. The maximum penetration depth of the additive substances into the capillaries of chips is limited by the maximally achievable extruder pressure. However, in spite of this complexity, the desired penetration of the chips with the additive material is still unsatisfactory.

In addition, the fabrication of wood-plastic composite materials with conventional extrusion processes has the following disadvantages:

Due to the limited penetration depth of the protective agents into the chips, protection of the wood-plastic composite material may no longer be possible when parts of the direct surface are removed.

The protective agents are also present in the plastic matrix which does not require this type of protection, which can add to the costs and cause an adverse effect on the plastic matrix depending on the type of the protective agent.

Changes in the chip quality can lead to a strongly varying product quality. This is difficult to control in the extruder. Extruders are designed for specific products, so that variations in the product can only be compensated in a very limited way by changing operating parameters.

Dosing ranges for additive materials are determined by the design, so that significant changes caused by altered requirements for the additional substances must have similar physical properties, such as density, aggregate state. As a result, necessary changes in the production process, such as a transition from using color pigments to liquid dyes, can no longer be made without substantial changes to the extruder.

The quality of protection of the chips can rarely be affected.

A process for controlled addition and subsequent uniform distribution of the additive substances in the wood chip may only be slightly controllable or not at all.

Possible treatment of the wood chip is an essential quality criterion for the final product, which is subject to significant demands regarding its haptics and visual appearance.

DE 197 57 071 A1 discloses a method for hydrophobing of chip material, which can be arranged upstream of the production of wood-plastic composite materials. Specifically designed double-screw systems are used where the chips are subjected to very high pressures and high temperatures for hydrophobing. In addition to the very complex system technology, the process environment in this solution produces wood chips with a noticeably inferior quality. Wood material heated to a high-temperature has a number of properties that are unsuitable for the subsequent use in a high-quality application, such as decreased rigidity and toughness.

DE 100 30 363 B4 describes a three-stage process for producing a fiber composite, wherein a first process step fiber compounds and/or wood particles are dried in a continuously operating throughput dryer and optionally mixed with an aqueous dispersion of melamine resin nanoparticles and/or fill material and modified such that in a second process step the dried and optionally modified fiber materials and/or wood particles are processed in a continuously operating throughput mixer, with the reaction mixture that is discharged as a strand or in granular form finally being supplied to extrusion or injection molding processing in a third process step.

Disadvantageously, this process, in spite of using complicated three-stage processing, does not achieve a high-quality treatment of wood chips with almost complete penetration of the chips by the additive substances. A substantial fraction of the additive substances which have not penetrated into the chips may remain in the plastic matrix of the finished product, which can adversely affect the product quality.

Because of the disadvantages of the conventional solutions, it is an object to develop a method for producing a high-quality wood-plastic composite material where the wood chips are colored throughout and as completely as possible, or impregnated with active substances in order to modify their properties. Specific demands on the extrusion pressure and the material supply to the extruder system should be avoided.

The object is solved with the invention with a method for producing a wood-plastic composite material having the features of claim 1. The dependent claims 2 and 7 describe particularly advantageous embodiment of the method.

The core off the method off the invention lies in drying and treatment of the wood chips with color-forming substances and/or active substances in any process stage arranged upstream of the extrusion in a fluidized bent apparatus.

This produces a particularly gentle and particularly intensive drying and penetration of the wood chips with color. By treating the wood chips in a fluidized bed, one can work during the treatment phase with a significant excess of color-forming substances and/or active substances so that rapid and intensive penetration of all wood chips can be achieved, without introducing color-forming substances and/or active substances that are not bound in or on the chips into the following extrusion process step of treated wood chips, plastic and optionally additional auxiliary substances and additive substances used for the wood-plastic composite material. This completely and very uniformly introduces the color-forming substances and/or active substances into each wood chip up to the maximum capacity, because as a result of the fluidization in the fluidized bed, each individual wood chip is reached by particles of the color-forming substance and/or active substance that are deposited on the wood chip and deeply penetrate the wood chip. After processing, wood chips can be removed from the fluidized bed according to their size—within a predetermined size spectrum—and extruded in the subsequent process step with plastic. Introduction of color-forming substances and/or active substances that did not penetrate into the wood chips into the following process step for extruding the wood-plastic composite material can be almost entirely prevented. There is also no undesirable incorporation of color-forming substances and/or active substances into the plastic matrix.

The separation of a fine fraction of a predetermined size range during removal of the wood chips from the fluidized bed is also of particular importance. Only with chips with a predetermined size spectrum are then incorporated in the wood-plastic composite material, thus significantly improving its quality.

In addition, the size classification quality of the wood chips can be less stringent, making the starting material for the production a wood-plastic composite material less expensive.

Moreover, the fluidized bed technology facilitates process control for drying and treatment of the wood chips. The process parameters (quantity, temperature, moisture content and flow velocity of the fluidizing gas and/or duration of the treatment in the fluidized bed) can be easily changed over a wide range by changing the parameters of the starting materials, i.e., of the wood chips as well as of the color-forming substances and active substances.

The invention as well as significant advantages of the invention will now be described in more detail with reference to the following exemplary embodiment.

For producing a wood-plastic composite material, in a first process step, wood chips having a initial moisture content in a range from 10% to 14% are dried at an inlet temperature of the fluidizing gas in a range from 40° C. to 100° C., preferably in a range from 60° C. to 80° C., to a residual moisture content in a range from 0.5% to 10%, preferably in a range from 1% to 3%.

After drying, color-forming substances and active substances against rot and pests as well as fire retardants and UV protection are injected into the fluidized bed. The quantity of the injected color-forming substances is in a range between 4 mass % and 6 mass %, with reference to the dry matter mass of the wood chips. The total quantity of the active substances is in a range between 1 mass % and 2 mass %, with reference to the dry matter mass of the wood chips. The color-forming substances and the active substances are injected in the liquid phase, with the liquid phase substances injected for a duration of about 20 minutes. The temperature in the fluidized bed is here in a range of 35° C. to 40° C. The fluidizing gas is introduced with a temperature in a range from 60° C. to 80° C.

Thereafter, post-drying is performed for a duration in a range from 2 to 10 minutes. The temperature in the fluidized bed increases during this time to values in a range from 60° C. to 75° C., wherein the fluidizing gas continues to be introduced with a temperature in a range from 60° C. to 80° C.

The wood chips can be treated in a fluidized bed, as described above, in a fluidized bed apparatus designed for discontinuous operation; however, the wood chips can also be treated in a continuously operating fluidized bed trough.

The treated and dried wood chips are removed from the fluidized bed apparatus sorted by size, whereby only wood chips having sizes ≧0.1 mm are removed and supplied to the second process step.

In the second process step, the sized wood chips, plastic and lubricants are supplied to an extruder. The wood-plastic composite material is extruded with the extruder in a conventional manner.

The aforedescribed process for producing a wood-plastic composite material has the following significant advantages:

The extruder technology, in particular the mixing stage and process control in the extruder, are simplified because only the supply of the treated and sized wood chips, of the plastic and the lubricant and other optional auxiliary substances for the extruder process need to be controlled.

In the wood-plastic composite material produced with the invention, the color-forming substances and active substances are only disposed in the wood chips and not into plastic matrix, where they would have no functionality and only increase cost. It will be understood that this does not prevent the addition of color to the plastic matrix. If this is desired or necessary, color-forming substances can also be introduced into the plastic matrix. These can be identical to or different from the color-forming substances for the wood chips.

Treatment of the wood chips with color-forming substances and active substances is very intense and results in an almost complete penetration of the entire chip volume of all treated wood chips. Optional machining of the wood-plastic composite material may therefore not expose untreated chip regions which may adversely affect the quality.

By separating the wood chips according to their size after treatment in the fluidized bed, only wood chips within a predetermined size range are incorporated in the wood-plastic composite material, which significantly improves quality. In particular, the separation of the fine fraction of wood chips and optionally of the color-forming substances and active substances produces a very advantageous effect.

The requirements on the quality of the wood chips employed with the process of the invention can be relaxed, making this starting material for the production of the wood-plastic composite material less expensive.

Treatment of the chips in the fluidized bed also improves their flow characteristic, which likewise simplifies process control in the extruder.

With the method of the invention, wood-plastic composite materials with a significantly improved quality can be produced, while simultaneously simplifying the extrusion process and the process control and reducing the cost of the wood starting material. Treatment of the chips in the fluidized bed is essential for an almost complete penetration of all wood chips with color-forming substances and active substances.

The process for producing a wood-plastic composite material described in more detail in the exemplary embodiment represents a particularly advantageous embodiment of the invention and does not restrict the invention to the disclosed parameters. 

1-7. (canceled)
 8. A method for producing a wood-plastic composite material, comprising the steps of drying wood chips in a fluidized bed apparatus to a residual moisture content below 10% processing the wood ships by adding color-forming substances and/or active substances, removing the wood chips from the fluidized bed apparatus, and feeding the processed wood chips with plastic and to an extruder, producing wood-plastic-composite material.
 9. The method according to claim 8, further comprising the step of discharging the wood chips from the fluidized bed according to size with a predetermined size distribution.
 10. The method according to claim 9, wherein the wood chips are discharged according to size with dimensions of ≧0.1 mm.
 11. The method according to claim 9, wherein the wood chips are processed in the fluidized bed apparatus with an excess of color-forming materials and/or active substances necessary for complete coloration and/or penetration of the wood chips.
 12. The method according to claim 8, wherein fluidizing in the fluidized bed apparatus is accomplished with a fluidizing gas at a temperature in a range from 40° C. to 100° C., and dried to a residual moisture content in a range from 0.5% to 10%, followed by an injection of color-forming substances and/or active substances into the fluidized bed, wherein the wood chips are treated for a time between 10 minutes to 30 minutes at a fluidized bed temperature between 35° C. to 40° C., wherein the entry temperature of the fluidizing gas is in a range between 75° C. to 85° C., and wherein the temperature of the fluidized bed is increased to 60° C. to 35° C. for a duration between 2 minutes and 10 minutes for post-drying the wood chips, and wherein the dried and processed wood chips are removed from the fluidized bed apparatus.
 13. The method according to claim 8, wherein the color-forming substances and/or active substances are introduced in the fluidized bed in a liquid dispersed form.
 14. The method according to claim 12, wherein the temperature of the fluidizing gas is in a range between 60° C. to 8020 C.
 15. The method according to claim 12, wherein the drying is accomplished to a residual moisture content in the range between 1% to 3%.
 16. The method according to claim 8, wherein additional auxiliary substances or additives are added to the processed wood chips and plastic before it is fed to the extruder.
 17. The method according to claim 14, wherein the quantity of the color-forming substances injected into the fluidized bed is in a range between 4 mass % and 6 mass %.
 18. The method according to claim 14, wherein the quantity of the active substances injected into the fluidized bed is in a range between 1 mass % and 2 mass %. 